The focus of this project was to describe in the rat pineal the sequale of biochemical events regulating cAMP levels following termination of beta-receptor stimulation. Present results show that cAMP in rat pineals respond in a biphasic manner to diminished receptor stimulation. Levels of cAMP decreased significantly within 30 to 60 sec following exposure of dark-adapted animals to light or administration of propranolol (10 mg/kg i.v.). Within 5 min. cAMP levels had significantly increased compared to 20 sec time points. While there were no changes in either cytoplasmic PDE or calmodulin levels, adenylate cyclase activity did increase at the same time that cAMP increased. Thus apparently cyclase activation could support the observed rise in cAMP levels. Present results are consistent with previous observations using dispersed pinealocytes and showing that diminished receptor stimulation leads to a fall in cAMP levels which precedes an abrupt drop in N-acetyltransferase activity. However, further investigation is necessary to link the change in cAMP with the fall in NAT.